The player or players who will sacrifice lands is determined only as Tectonic Hellion's triggered ability begins to resolve.

8/23/2019

If more than one player controls the most lands, each of those players sacrifices two lands. If everyone controls the same number of lands, everyone sacrifices two lands.

8/23/2019

If more than one player will sacrifice lands as Tectonic Hellion's triggered ability resolves, first the player whose turn it is chooses two lands they control (if they control the most lands), then each other player in turn order does the same if they control the most lands, knowing the choices made before them. Then all the chosen lands are sacrificed at the same time.